Crude oil from individual oil wells is transported through flow lines to individual vessels generally located at or near the well site. From there, the oil may undergo further separation, treating, heating, dehydrating, compressing, blending, pumping or other processing activities before being transported downstream, via truck, pipeline, rail car or other transportation method. It can enter into gathering lines to a point where it enters a common carrier pipeline and eventually arrives at the refinery. When custody of oil is transferred from one owner or processing point to another during this process, it is useful to know the composition and physical properties of the oil being transferred for a variety of reasons, including, but not limited to, the sale price of the crude, the desired final destination for refining, and the timing of the delivery, safety and material handling requirements for storage/transportation.
The current standard for assessing the “quality” of oil in the field is determining its relative density or API gravity, vapor pressure, water content, and basic solids and water content (“BS&W”). Upper limits are set on the BS&W content and vapor pressure; and, the pricing of the oil is generally related to the API gravity number. In the past, oil produced from conventionally drilled wells with certain API numbers would generally all have similar compositional values. However, this general correlation is increasingly being challenged. One of the biggest contributing factors challenging this historical assumption is new production from unconventional shale formations. This shale crude oil tends to be much lighter (higher API gravity number) and can vary significantly in composition from well to well, and even from within an individual well. The amount of time the crude is stored in a tank can also have an effect on the composition. As the crude sits, it becomes “weathered”, meaning some of the lighter components have flashed off and been vented, flared, or injected into the gas phase pipeline leaving the production facility. The increasing and changing sources of production have also resulted in the comingling of various streams from different sources to generate oil with specific desired properties, resulting in much different hydrocarbon compositions of blended oil than previously seen.
It would be useful to know the constituent compositions of the oil entering the common carrier line from each gathering line in real time so that the composition of the oil in the common carrier line can be monitored and adjusted as desired to meet specifications such as those imposed by individual refineries. This would enable the prevention of practices such as “bar-bell-ing” where a specified API gravity number is obtained through the combination of light end hydrocarbons with much heavier “sludge” hydrocarbons without the highly desirable intermediates such as C7-C12. It would also allow for blending and marketing of very specific oil compositions to take place even upstream or midstream, well before selling to the refineries. Finally, it would allow the refineries to significantly increase their production efficiency and thus profit margins by monitoring incoming oil composition and paying more or less for specific blends, rather than just relying on API gravity numbers.
There is a need, therefore, for a method and system for determining the energy content, physical properties and the constituent composition of crude oil entering the common carrier lines or other transportation systems such as rail cars or trucks from each gathering line, or passing through custody transfer points, in real time so that the composition of the oil can be monitored and adjusted as desired.